This invention relates to a photothermographic material and, more particularly, to a photothermographic material for scanners, image setters or the like suitable for photomechanical processes. More specifically, this invention relates to a photothermographic material that can obtain such images optimal for photomechanical processes as having a high Dmax (maximum density), a low fog, a good coated surface condition, and less surface defects such as repellency, coating lines or the like.
A large number of photosensitive materials having a photosensitive layer on a support for forming images upon imagewise exposure have been known. Among them, as a system for rendering image forming means simplified, a technology for forming images by heat development is utilized.
In recent years, reduction of the amount of waste processing solutions is strongly demanded in the field of photomechanical processes from the standpoint of environmental preservation and space savings. To cope with this, techniques are needed in relation to heat photosensitive developable materials for use in photomechanical processes, which can be effectively exposed by a laser scanner or laser image setter and can form clear black images having high resolution and sharpness. Such photothermographic materials can provide a heat development processing system, without use of solution-type processing chemicals, simpler and free from incurring environmental destruction.
Methods for forming an image by heat development are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075 and D. Morgan and B. Shely, Imaging Processes and Materials, xe2x80x9cThermally Processed Silver Systemsxe2x80x9d A, 8th ed., page 2, compiled by Sturge, V. Walworth and A. Shepp, Neblette (1969). The photosensitive material described in the above contains a light-insensitive silver source (e.g., organic silver salt) capable of reduction, a photocatalyst (e.g., silver halide) in a catalytic activity amount, and a reducing agent for silver, which are usually dispersed in an organic binder matrix. This photosensitive material is stable at room temperature, but, when it is heated at a high temperature (e.g., 80xc2x0 C. or higher) after the exposure, silver is produced through an oxidation-reduction reaction between the silver source (which functions as an oxidizing agent) capable of reduction and the reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image generated upon exposure. The silver produced by the reaction of the silver salt capable of reduction in the exposure region provides a black image and this presents a contrast to the non-exposure region. Thus, an image is formed.
In most of conventional photothermographic materials, the photosensitive layer is formed by coating a coating liquid having a solvent as an organic solvent such as toluene, methyl ethyl ketone (MEK), methanol, and the like. Use of such organic solvents as a solvent not only adversely affects human bodies during manufacturing processes but also is disadvantageous in term of costs due to recycling the solvents and others.
To cope with this, a method has been considered in which a photosensitive layer is formed using a coating liquid of a water solvent not having the above problem. For example, Japanese Unexamined Patent Publication (KOKAI) Showa (hereinafter referred to as xe2x80x9cJP-A-xe2x80x9d) Nos. 49-52, 626 and 53-116,144, and the like set forth an example that gelatin is used as a binder. Also, JP-A-50-151,138 sets forth an example that a polyvinyl alcohol is used as a binder. In JP-A-60-28,737, an example that a gelatin and a polyvinyl alcohol are used together is described. In addition, as another example other than the above examples, JP-A-58-28,737 sets forth an example of a photosensitive layer that a water-soluble polyvinyl acetal is used as a binder.
Such a binder surely allows to form the photosensitive layer in use of a coating liquid with a water solvent, thereby making such use advantageous in terms of environments and costs.
However, if the polymer such as gelatin, polyvinyl alcohol, water-soluble polyvinyl acetal, and so on is used as the binder, not only that a silver tone at the developed portion is rendered brown or yellow which is so deviated from black, originally favored color, or products having considerably diminished values are only obtained such that the blackened concentration at a light exposed section is low while the concentration at an unexposed portion is high, but also that the binder has a bad solubility with an organic silver salt, thereby rendering coatings unavailable with a surface having a practically durable quality.
European Patent No. 762,196, and JP-A-9-90,550 disclose that photosensitive silver halide particles used for the photothermographic materials contain VII-group or VIII-group metal ions or metal complex ions and that high contrast photographic characteristics can be obtained by containing hydrazine derivatives in the photosensitive materials. However, if the binder used in the coating liquid of the above water solvent and a nucleation agent such as hydrazine are concurrently used, a high contrast image can be obtained, but at the same time there raise problems such that fog may likely occur.
On the other hand, it is possible that lowering pH of a surface of an outmost layer on an image forming layer side reduces the fog to some extent, but there occurs surface defects such as repellency or coating lines, and these defects avoid applications that require a large screen such as, especially, photomechanical process from being practically used.
Accordingly, an object of the present invention is to solve these problems of conventional art.
That is, an object of the present invention is to provide, advantageously in terms of environments and costs, a photothermographic material suitable, especially for such as scanners, image setters or the like, for photomechanical processes, the heat photosensitive material being capable of obtaining such images optimal for photomechanical processes as having a high Dmax (maximum density), a low fog, a good coated surface condition, and less surface defects such as repellency, coating lines or the like.
As a result of diligent study for accomplishing these objects, the present inventors found that an excellent photothermographic material achieving desired advantages can be provided by forming a layer containing specific compounds on an outer side of the image forming layer, and the present invention has been accomplished.
That is, the present invention is to provide a photothermographic material having on a support a non-photosensitive sliver salt, a photosensitive sliver halide, a nucleation agent, and a binder, comprising, on an outer side of the image forming layer, a layer including:
one or more organic acid compounds represented by following Formula (1), 
xe2x80x83(where T represents a univalent substituent and k1 represents an integer of from 0 to 4; in the case of k1 greater than 2, each of plurality of T may be the same or different one another and may be combined to form a ring; L1 and L2 each represents a bivalent linking group; n1 and n2 each independently represents an integer of from 0 to 30),
and one or more compounds represented by following Formula (2), Formula (2)
Rxe2x88x92(A)nxe2x88x92Y
xe2x80x83(where R represents an alkyl group, an alkenyl group or an aryl group which may be substituted or unsubstituted and have 6 to 30 carbon atoms; A represents a bivalent linking group; n represents an integer of from 0 to 50; Y represents xe2x80x94SO3M or xe2x80x94OSO3M; M represents a hydrogen atom, an alkali metal atom, an alkaline-earth metal atom, an ammonium group or a lower alkylamine).
In the photothermographic material according to the present invention, the image forming layer preferably contains 50% by weight or higher of a polymer latex, as a binder, having a glass transition temperature of xe2x88x9230xc2x0 C. to 40xc2x0 C. with respect to the whole binder.
Further, in the photothermographic material, the nucleation agent is at least one compound selected from a group consisting of:
a substituted alkene derivative represented by following Formula (3), 
xe2x80x83(where R1, R2 and R3 each independently represents a hydrogen atom or a substituent, Z represents an electron withdrawing group or a silyl group, and R1 and Z, R2 and R3, R1 and R2, and/or R3 and Z may be combined with each other to form a ring),
a substituted isoxazole derivative represented by following Formula (4), 
xe2x80x83(where R4 represents a substituent), and an acetal compound represented by following Formula (5), 
(where X and Y each independently represents a hydrogen atom or a substituent, A and B each independently represents an alkoxyl group, an alkylthio group, an alkylamino group, an aryloxy group, an arylthio group, an anilino group, a heterocyclic oxy group, a heterocyclic thio group or a heterocyclic amino group, and each of X and Y, as well as, A and B may be combined with each other to form a ring structure).
In addition, pH of a surface of an outmost layer on the image forming layer side of the photothermographic material according to the present invention is preferably 6 or less.